For most users we recommend using Thunderbird. More detailed instructions on how to configure this are included on our general mail support pages.
For users accessing Unix via a terminal application (such as ssh), Pine is a popular mail program, and can be run simply by typing
pine. A good place to find information on Pine is the Pine FAQ. Another good mail program is mutt (Mutt Manual).
Some mail programs use a separate program for composing (editing) messages, and you may wish to choose your preferred editor rather than use the unfriendly system default (vi). This is covered in the page on text editors.
You might also want to see our introduction to email web page for more basic information.
These features are both set up through the Physics webmail system. The “vacation” command and “.forward” file are not used.
For secure email using Thunderbird see FAQ: Thunderbird Email Encryption.
For users accessing via the terminal we have both GnuPG and PGP v5.0 installed. GnuPG is probably a better choice for most purposes. There's a good introduction to using GnuPG with mutt at http://codesorcery.net/mutt/mutt-gnupg-howto.